1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an apparatus and method for driving an organic light emitting display, which can minimize or prevent a raindrop phenomenon and an afterimage.
2. Discussion of the Related Art
An organic light emitting display is a self-luminous display that emits light by electrically exciting a fluorescent organic compound, and displays an image by driving N×M organic light emitting diodes (OLEDs).
There are two driving methods for the organic light emitting display, that is, a passive matrix (PM) method and an active matrix (AM) method. In the case of the PM method, anode electrodes and cathode electrodes are formed perpendicular to one another and the display is driven by selecting lines. In the case of the AM method, transistors and capacitors are connected to pixel electrodes formed of indium tin oxide (ITO) and the display is driven by maintaining voltages at the pixel electrodes using the capacitors.
FIG. 1 is a block diagram of a related art organic light emitting display. Referring to FIG. 1, the related art organic light emitting display includes a timing controller 10 for generating control signals, a scan driver 11 for sequentially supplying a scan signal “Vs” in response to a control signal generated from the timing controller 10, a data driver 12 for supplying a data signal “Vdata” in response to a control signal generated from the timing controller 10, a power supply 13 for supplying a power voltage “VDD”, and a pixel unit 14 for driving an OLED according to the scan signal Vs and the data signal Vdata. The pixel unit 14 further includes a plurality of pixels 15 arranged in a matrix pattern.
FIG. 2 is a circuit diagram of a pixel shown in FIG. 1. Referring to FIG. 2, a pixel 15 includes a first transistor M1 connected to a data line 17 to be turned on by the scan signal Vs, a second transistor M2 connected to the first transistor M1 to be turned on by the data signal Vdata, a capacitor C connected to the first transistor M1 to maintain the data signal Vdata during a predetermined period (for example, one frame), and an OLED connected between the second transistor M2 and a power supply line 18 to emit light by a driving current of the second transistor M2.
When the first transistor M1 is turned on by the scan signal Vs, the data signal Vdata is charged in the capacitor C. Also, when the second transistor M2 is turned on by the data signal Vdata, the OLED emits light by the driving current of the second transistor M2. The OLED emits light by a given driving current, wherein the strength of the given driving current is proportional to the strength of the data signal Vdata. That is, when a data signal Vdata applied to the second transistor M2 becomes larger, the strength (that is, luminance) of light emitted from the OLED becomes larger because the driving current becomes larger. On the contrary, when the data signal Vdata becomes smaller, the strength of light emitted from the OLED becomes smaller because the driving current becomes smaller. Accordingly, the light strength (luminance) of the OLED can be controlled according to the strength of the data signal Vdata.
Each pixel 15 of the pixel unit 14 is driven by the scan signal Vs of the scan driver 11 and the data signal Vdata of the data driver 12. The scan driver 11 and the data driver 12 are driven by the timing controller 10. Accordingly, when a control signal is generated from the timing controller 10, each pixel 15 of the pixel unit 14 can be driven. If a control signal is not generated from the timing controller 10, the pixel unit 14 is not driven. The power supply 13 constantly supplies the power voltage VDD to the pixel unit 14, irrespective of the timing controller 10.
That is, even when a control signal is not generated from the timing controller 10 and thus the pixel unit 14 is not driven, the power voltage VDD is continuously supplied to the pixel unit 14. In this case, each pixel 15 of the pixel unit 14 may or may not emit light. That is, even when the first and second transistors M1 and M2 are turned off, the OLED may emit light due to the power voltage VDD or may not emit light. If the OLED is driven by the power voltage VDD even when the pixel unit 14 is not driven, an undesirable raindrop phenomenon or afterimage may be generated. That is, if some of the pixels 15 in the pixel unit 14 emit light due to the power voltage VDD even when the pixel unit is not driven, a raindrop phenomenon or an after image may be generated.